Running requires the muscles of the leg to lift the leg up and then pull the leg down. The speed and rate of the leg going up must match the speed of the leg going down. Following the premise that a chain is as strong as its weakest link, the runner's rate of turnover (strides per second) is limited by whichever muscle group is the weakest or slowest (the muscle group that lifts the leg or the muscle group that lowers the leg).
The lower back-glute-ham group of muscles that drives the legs down includes much bigger muscles than the hip flexor group which pulls the leg up. Additionally, the lowering of the leg is assisted by gravity while the hip flexor group has to work against gravity to get its work done to pull the leg forward and up. It is for these reasons that the hip flexor group is the limiting factor in stride frequency, and requires the most developmental focus if increased speed is the goal. The sprinter training device of the invention is designed with this in mind, to increase the athlete's stride frequency by increasing the power generated by the hip flexor group. It achieves this by altering the run cycle so that the hip flexor group is targeted for muscle development, much like lifting weights for the hip flexor group.
The following formula is a description of the physics of hip flexor movement.
Hip Flexor Power Development:                A. The leg has a defined weight.        B. The swinging leg, on its way backward, moves at a measurable speed.        C. Weight (load)×Speed=Power.        
The weight of the leg multiplied by the speed that the leg is moving will determine the power required to stop the limb and change its direction and move it forward. By forcing an athlete to run faster than the athlete's unassisted speed, the speed factor of the power equation is modified and therefore, the athlete's power is increased. It is in the increase in power that allows the athlete to achieve higher speeds and thus have greater speed endurance.
Fast Twitch Maintenance:
By causing the muscles of the leg to be energized in a more rapid sequence, the nervous system of the sprinter's leg is adapted to faster firing of the muscles involved in the run cycle.
Nervous Adaption:
If a runner ran downhill for 30 meters, he/she would most certainly run faster than a flying 30 meter run (natural, unassisted maximum velocity). This is because the nervous system is much more cooperative when the muscles that are innervated are relaxed.
However running downhill has been shown to not be the optimal way to increase speed. When running downhill, the runner is assisted by gravity, which tends to continue to accelerate the runner to his/her terminal speed, a condition in which relaxation is not possible. The pattern of nerve innervation and muscle firing sequence is altered from that of the unassisted running cycle. When the maximum over speed rate is achieved when running downhill, the runner adapts his run cycle, muscle innervation sequence and muscle firing pattern to cause the runner to lean backwards, apply breaking pressure to the feet when they are on the ground to absorb increased foot strike from the downhill posture, and in many subtle ways to change the downhill run cycle from that of the unassisted run cycle.
Downhill running puts the ankle joint at an unnatural position concerning the initial heel contact. It extends the ankle joint, negating its ability to add impulse to the body. By contrast, the machine of the invention allows the runner to have the same ankle position that he would use if he were unassisted. Because the runner is not at his or her maximum velocity, the runner does not “put on the brakes” nor change his posture, shifting the center of gravity backward.
Natural Running Curve:
All runners have to overcome the load of the body at the beginning of any run. Use of the motor of the device to pull the runner backward at the start of the sprint adds a slight load to the runner in order to exaggerate the runner's normal load (inertia). Running downhill or with the assistance of a bungee cord, for example, removes this normal load and is counterproductive to acceleration training.